In a typical cellular radio system, wireless terminals (also known as radio devices, mobile stations and/or user equipments (UEs)) communicate via a radio access network (RAN) to one or more core networks (CN). The wireless terminals can be mobile stations or user equipments (UE) such as mobile telephones (cellular telephones) and laptops with wireless capability (e.g., mobile termination), and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data via radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called Node B (NB) or evolved Node B (eNode B or eNB). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments (UE) within range of the base stations.
In some versions (particularly earlier versions) of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto.
The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. Universal Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected directly to a core network rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are performed by the radio base station nodes. As such, the radio access network (RAN) of an LTE system has an essentially flat architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
The following description, for purposes of explanation, refers to LTE, WCDMA, UTRAN or evolved UTRAN (E-UTRAN or eUTRAN). This does however not limit the applicability to other technologies.
Access Control
UTRAN and LTE offer so called access control mechanisms by which the network can prevent UEs from accessing the network. Obviously, this is desirable when the network experiences an unsustainable high load. This may be the case if, due to access burst, there are no further radio or processing resources in the eNB/NB available to fulfill the service requirements of all UEs that desire to transmit data. In such situations it is preferable to prevent additional (IDLE) UEs from accessing the network and thereby to offer sufficient quality of experience to already connected UEs.
This is known as access barring. Similarly, the NW may decide to reject or release already connected (RRC CONNECTED) UEs from the NW. This is known as RRC CONNECTION REJECT or RRC CONNECTION RELEASE.
Standardized access barring schemes allow to block certain UEs while still permitting others to access the network. In particular the specifications allow distinguishing mobile terminating calls, mobile originating calls, emergency calls, mobile originating signaling, mobile originating CS fall-back, special access classes (Access Classes 11-15), and extended access barring (for lower priority traffic). Furthermore, there exist means to prevent UEs from performing access for multimedia telephony (MMTEL) MMTEL-Voice or MMTEL-Video. In LTE and UMTS the access barring schemes are currently only applicable for UEs in IDLE mode. That means, a UE that is already in RRC CONNECTED may access the network even if the current cell indicates that access is barred. It has recently been proposed in 3GPP to extend access barring so that it is also applicable to UEs in RRC CONNECTED. Such investigations are on-going.
QoS in LTE and UMTS
LTE and UMTS make use of so called quality of service (QoS) Classes (QCI) that were introduced to achieve an abstraction between services and their quality of service requirements on one side and the RAN and its scheduling QoS logic on other side.
The core network (CN) decides how many different levels of quality of service need to be distinguished in the RAN (and potentially in the transport and core network) and sets up a corresponding number of radio bearers for each UE. The core network also defines so-called packet filters which allow the non-access-stratum (NAS) layer in the UE and the core network (in a gateway of the CN) to decide which packet to map onto which bearer. This filtering is primarily done based on source and destination IP address and port number. It is therefore flexible so that the network can easily map different kinds of applications to different bearers.
With this approach, the access stratum (in the RAN) only distinguishes bearers while it does not need to be service aware. All data mapped (by packet filters in the UE) onto one bearer is expected to get the same QoS treatment by the RAN and the UE. The packet treatment is determined by the core network which sets the QoS class indicator (QCI) for each established bearer.
In the RAN, evolved packet system (EPS) bearers are mapped to data radio bearers (DRBs) having logical channel identity (LCI). Scheduling and prioritization of data packets in the RAN is done based on logical channels in radio resource control (RRC) Connected mode.
Random Access
In modern cellular radio systems, the radio network has a strict control on the behavior of the UE. Uplink (UL) transmission parameters like frequency, timing, and power are regulated via downlink (DL) control signaling from the base station to the UE.
At power-on or after a long standby time, the UE is not synchronized in the uplink. The UE can derive an uplink frequency and power estimate from the downlink control signals. However, a timing estimate is difficult to make since the round-trip propagation delay between the base station, e.g. eNodeB radio base station in LTE, and the UE is unknown. So even if UE uplink timing is synchronized to the downlink, it may arrive too late at the eNodeB receiver because of the propagation delays. Therefore, before commencing traffic, the UE has to carry out a Random Access (RA) procedure to the network. After the RA, the eNodeB can estimate the timing misalignment of the UE uplink and send a correction message. During the RA, uplink parameters like timing and power are not very accurate. This poses extra challenges to the dimensioning of a RA procedure.
Usually, a Physical Random Access Channel (PRACH) is provided for the UE to request access to the network. An RA preamble is used which is based on a specific sequence with good auto-correlation. Because multiple UEs can request access at the same time, collisions may occur between requesting UEs. A contention resolution scheme then has to be implemented to separate the UE transmissions. To distinguish between different UEs performing RA, typically many different preambles exist. A UE performing RA randomly picks a preamble out of a pool of preambles and transmits it. The preamble represents a random UE identity (ID) which can be used by the eNodeB when granting the UE access to the network. The eNodeB receiver can resolve RA attempts performed with different preambles and send a response message to each UE using the corresponding random UE IDs. In case that multiple UEs simultaneously use the same preamble, a collision occurs and most likely the RA attempts are not successful since the eNodeB cannot distinguish between the two UEs with the same random UE ID.
To minimize the probability of collision, the set of available sequences, i.e. preambles, should be large. In LTE, the number of provided sequences per cell and RA opportunity is 64. Preambles assigned to adjacent cells are typically different to insure that a RA in one cell does not trigger any RA events in a neighboring cell.
LTE defines different RA configurations that differ in the amount of offered RA opportunities. A RA opportunity is approximately 1 MHz wide and either 1, 2, or 3 milliseconds (ms) long within which the UE can transmit the RA preamble. In the configuration with the lowest number of opportunities, one opportunity is offered every second radio frame, i.e. every 20 ms. On the other extreme the configuration with the highest density of RA opportunities offers one RA opportunity every subframe, i.e. every ms.
The eNodeB receiver listens at all RA opportunities to detect preambles. In case a preamble is successfully detected, a RA response that includes the number of the detected preamble is sent in a special message on the DL. A UE that has recently performed a RA attempt is listening on the DL within a certain time window after the preamble has been sent, to receive a RA response. In case of a successful reception of the RA response, the UE continues with the RA procedure steps for contention resolution. In case no RA response is received within the specified window, a new attempt is made. Also, if the contention resolution does not indicate that the UE won the contention, a new attempt is made. The power of this new attempt preamble transmission is increased by a configured step size relative to the previous attempt. Depending on the back-off parameter in the UE, the UE may immediately re-try or wait for a random time depending on the configured back-off time prior a new attempt.
In addition to a contention-based RA procedure, LTE supports a contention-free variety of the RA procedure in which the eNodeB directs the UE to use a specific preamble not simultaneously used by any other UE in the same cell and the steps of contention resolution are not needed.
If the number of unsuccessful RA attempts exceeds a configured threshold, lower layers in the UE terminal indicate to higher layers that there is a random access problem. In LTE, the Medium Access Control (MAC) layer indicates a random access problem to a Radio Resource Control (RRC) and continue random access attempts, i.e. the random access procedure is not stopped. Depending on higher layer state and conditions, higher layers declare a radio link failure or let the random access procedure continue until one or more higher layer procedure timers expire/time-out or a stopping condition is met.
System Information
In mobile cellular networks, system information is broadcasted in each cell and contains various information needed for the user equipment to both access the cell and properly operate within the cell. In LTE, system information is structured into one Master Information Block (MIB) and a number of (currently 16) System Information Blocks (SIBs), referred to as SIB1 through SIB16. The MIB is transmitted in subframe #0 of the LTE radio frame. In contrast to the MIB, SIBs are schedulable in both time and frequency (SIB2 through SIB16), or in frequency only (SIB1). FIG. 1 illustrates the timing of transmissions of MIB and SIB1. In LTE, SIB1 is transmitted in subframe #5 of even radio frames.
The timing of other SIBs (SIB2 through SIB16) is provided with the scheduling list field provided in SIB1. Hence, to acquire SIBs other than SIB1, the UE needs to first acquire SIB1.
System Information comprise access control information, e.g., cell barring parameters in SIB1, access class barring parameters in SIB2 and extended access barring parameters in SIB14.
System information (SI) can be updated at regular points in time by the network, wherein a concept of a modification period is used. The network may only update SI at modification period boundaries. The length of the modification period is communicated to the UE, so it knows when the boundaries occur. When the network decides to update SI, it sends a paging message containing the systemInfoModification field. At the next modification period boundary, the updated SI is broadcasted, and the UEs may acquire the updated SI. There is also a field in SIB1 called systemInfoValueTag which is incremented each time the SI is updated, with some exceptions. This means that a UE can compare its previously acquired systemInfoValueTag with the one currently broadcasted to establish whether its SI is valid or not. In order for a UE to know whether its SI is valid or not, there are two legacy methods: the UE reads the paging messages, or the UE checks the systemInfoValueTag.
One exception to the system information update procedure is update of extended access barring parameters. Extended access barring parameters in SIB14 can be changed at any time and UEs can be informed about a change by means of a paging message including eab-ParamModification. An extended access barring capable UE is not expected to periodically check schedulingInfoList contained in SIB1.